What about Matthew 12:31-32 — speaking against the Holy Spirit will never be forgiven?
The short answer
Forgiveness requires repentance. "Blasphemy of the Spirit" is the unwillingness to repent and accept God's forgiveness; by rejecting grace, one can condemn oneself for eternity.
Forgiveness needs contrition
Contrition has first place among the penitent's acts: sorrow and hatred for sin with the resolve not to sin again (CCC 1451).
What the blasphemy is
Unwillingness to repent and accept God's forgiveness is what Jesus meant by blasphemy against the Spirit (Matthew 12:31-32). By rejecting grace in this life, one judges oneself and can even condemn oneself for eternity by rejecting the Spirit of love (CCC 679).